During automated fiber placement, a number of tows, or tapes, of fiber are typically fed in parallel fashion, as a band of tows, through a fiber placement head to a compaction roller which places the tows onto the surface of a tool. Each individual tow or tape is supplied from a spool, which is typically located in a stationary creel. The fiber placement head is typically movable, by an automatic controller, to orient the compaction roller properly for placing the band of tows onto the tool surface. Because the fiber placement head may move a considerable distance, in multiple directions, with respect to the creel, each of the fiber tows or tapes is typically routed through a number of redirects in traversing the distance between the creel and the head.
For various reasons, including preventing droop and tangling of the individual plys during certain motions of the fiber placement head, each fiber tow or tape is typically held in tension, between the creel and the fiber placement head. Tensioners inside the creel are often used for creating this tension.
During the automated fiber placement process, it is sometimes necessary to cut some of the tows to create openings for holes, windows, or other features in the part being constructed, or to narrow the band of tows that are being simultaneously laid down in parallel onto the tool surface. Conversely, tows may later be added back into the band of tows being laid down, when a wider band is desired.
In order to allow for the addition and cutting of tows, during the automated fiber placement process, fiber placement heads typically include various mechanisms for cutting and/or adding a tow. These mechanisms typically include a pinch cylinder, or other clamping mechanism, for holding the cut tow, or a tow not yet added to the band, in place at the fiber placement head, against the tension applied to the tow, so that the tow being held is not retracted away from the fiber placement head.
In the event that the pinch cylinder or tow clamping mechanism should fail, however, or a tow should break between the pinch cylinder and the creel, the tension on the tow will tend to draw the tow back toward the creel. This is an undesirable occurrence, because rethreading the tow through all of the redirects and feeding it back to the fiber placement head can be a complex and time-consuming process, during which the automated fiber placement operation must be shut down.
It is desirable, therefore, to provide an improved method and apparatus for holding a tow in place at the fiber placement head in the event of a malfunction of a pinch cylinder or breakage of the tow between the pinch cylinder and the creel.